1. Field of the Invention
The present invention relates to pyrolysis of hydrocarbons, and especially to a pyrolysis tube for enhancing the yield of olefins and a pyrolysis method thereof.
2. Description of the Related Art
Steam cracking of hydrocarbons is a reaction to produce olefins such as ethylene and propylene by using naphtha, diesel and the like as a resource. The main ingredients of the naphtha, diesel and the like are paraffin-based hydrocarbons.
The following conventional process is provided for steam cracking of hydrocarbons. The hydrocarbons and water are respectively vaporized, mixed together, and then the mixture thereof is preheated to about 600° C. In the next step, the mixture is decomposed thermally while being passed through a hot pyrolysis tube at a temperature above 800° C.
Since pyrolysis is an endothermic reaction, heat must be continually supplied from the outside to maintain a reaction. Therefore, the pyrolysis tube is heated by radiant heat transferred from a burner to continually feed heat. The mixture is passed through the heated pyrolysis tube at a high velocity of 100˜200 m/s and it resides therein for 0.2 to 0.4 seconds.
To improve a yield of olefin during pyrolysis, it is necessary to heat the mixture being passed through the pyrolysis tube quickly and uniformly, thereby preventing an undercracking and/or overcracking.
Since pyrolysis is an endothermic reaction as explained above, if the temperature gradient along the radius is high, hydrocarbons are thermally overcracked at the wall of the pyrolysis tube while it is thermally undercracked at the center of the pyrolysis tube, thereby yielding less olefin.
Moreover, the longer the residence time of the mixture in the pyrolysis tube, the more intensively secondary reactions of the olefins take place. The details of the secondary reactions of the olefins are as follows:
1) olefins are converted into aromatics by combining with each other;
2) olefins are converted into acetylene or diolefin by dehydrogenation; and
3) olefins are converted into methane by decomposition.
The secondary reactions of the olefin not only decrease the yield of the olefin, but they also increase a coking tendency in the pyrolysis tube, thereby lowering a heat transfer rate and shortening the longevity of the pyrolysis tube.
Therefore, since there should be a reduction in the residence time of the mixture in the pyrolysis tube, it is necessary to increase a fluid flow velocity or to use a pyrolysis tube of a small effective diameter.
In the former method of increasing the fluid flow velocity, if the residence time of the mixture in the pyrolysis tube is too short, the mixture cannot be provided with sufficient heat to react, and therefore some hydrocarbons are undercracked. As a result, there is a decrease in yield of olefin. Therefore, when pyrolysis tubes of the same effective diameter are used, a suitable residence time is necessary to maximize the yield of the olefin.
In the latter method of using a pyrolysis tube of a small effective diameter, since the temperature of the outer wall of the pyrolysis tube can be decreased because of relatively effective heat transfer, there is an advantage of reducing the coking tendency on the inner wall of the pyrolysis tube. However, since the diameter of the pyrolysis tube is small, depending on operating conditions, the cross-sectional area of the tube can be diminished more quickly by the coke, thereby necessitating frequent decoking of the tube. When the effective diameter of the pyrolysis tube is too small, or if the cross-sectional area of the tube is lessened because of the influence of the coke, there is an increase in pressure drop, thereby decreasing the yield of olefin with respect to the reaction mechanism.
Therefore, among the methods for manufacturing olefins by thermally cracking hydrocarbons, methods for increasing the yield of olefin with less coking tendency are provided.
U.S. Pat. No. 4,342,642 describes a method of producing a desired increase in heat flux without adversely increasing pressure drop. The method is accomplished by using a tube insert spaced away from the inner tube wall having outwardly extending arms or vanes that touch or almost touch the inner wall of the tube, and such a configuration has been found to provide a heat absorption surface that produces a desired increase in heat flux. The insert sub-divides a free internal cross-section of the tube into equal areas.
In the above invention, since the fluid in each sub-divided equal area cannot be mixed together, there is a limit as to uniformity of heating the mixture. In addition, since the coking area in the pyrolysis tube with the insert is larger than the area without an insert, the pressure drop caused by the coke adversely increases. Therefore, there is a problem in that the coke must be removed frequently.
French Patent No. 2,688,797 describes a method of heating the mixture uniformly in the pyrolysis tube. The method is accomplished by an insert with a long surface being installed along the axial direction in the rear end of the pyrolysis tube to improve the heat transfer rate and to develop turbulence.
Japanese laid-open Patent No. 9,292,191 provides a method of disposing a bar having fixed pins along the axial direction, thereby mixing the fluids passing through the pyrolysis tube.
The above French Patent and Japanese laid-open Patent have a common feature of using turbulence generated by pins or an insert within the pyrolysis tube. On the other hand, in both patents, assuming that the same quantity of mixture is passed through the pyrolysis tube with the insert as without, since the cross-sectional area of the pyrolysis tube decreases, there is a problem in that the velocity of the fluid flow in the pyrolysis tube increases. This also causes an increase of pressure drop in the pyrolysis tube.
In addition, Japanese laid-open Patent No. 11,199,876 describes a method of making protrusions in a pyrolysis tube. According to the above Japanese laid-open Patent, the fluid flow passing through the pyrolysis tube collides with the tube wall due to the protrusions, thereby preventing the fluid flow adjacent to the tube wall from stagnating and overheating. Therefore, it is possible to decrease the yield of coke.
According to the above specification, by mixing the fluid to the utmost, there is a decrease in coking of the tube and it is not necessary to remove the coke so frequently. However, it is described that there is little increase in the yield of ethylene.
In the conventional methods described above, heat transfer to the fluid passing through the pyrolysis tube is increased by reducing the effective diameter of the pyrolysis tube or increasing its effective surface area. Alternatively, the heat transfer rate is increased or the mixture is mixed uniformly by generating turbulence or swirl in the fluid flow passing through the pyrolysis tube due to pins or protrusions. Therefore, the method decreases the coking tendency.
However, the above methods have problems in that there is an increase in pressure drop or there is little improvement in yield of ethylene.